Multilayer electric contacts



US. Cl. 200-166 United States Patent 3,518,390 MULTILAYER ELECTRIC CONTACTS Takeshi Sasamoto, Tokyo, Japan, assignor to Nippon Electric Company, Limited, Tokyo, Japan Filed July 19, 1968, Ser. No. 746,078 Claims priority, application Japan, July 21, 1967, 42/ 16,922 Int. Cl. H0111 1/02 7 Claims ABSTRACT OF THE DISCLOSURE Selectively engageable electric contacts wherein at least one of the engaging contact surfaces is provided with a plurality of layers of metallic material upon a metallic base member in which the multiple layers are comprised of alternating layers of a first metal having a relatively small heat conductivity and a relatively high melting point as compared with the adjacent metallic layer wherein the metals are not diffused and wherein the metal having the lower melting point and lower heat conductivity constitutes the outermost layer in order to significantly reduce complementary buildup and pitting of the engaging contact surfaces and to assure even contact Wear after repeated use.

The present invention relates to improvements in the construction of electrical contacts for selectively closing and opening an electrical load circuit, and more particularly to improvements in the multilayer contact structure of such electrical contacts, and especially of those contacts employed in reed switches.

As is well-known in the switching contact art, electrical contacts are subject to electrical erosion, which may take the form of bridge transfer of molten metal between the contacts due to the current passing through the contacts. Such bridge transfer results in a buidup on one contact accompanied by the formation of a pit on the mating contact. Eventually such buildups can give rise to mechanical contact locking. In some cases, the contacts may become welded upon closure by excessive currents conducted through contact surfaces on which such buildups and pits have been formed. In either case, the switch will fail to open. In addition thereto, with an increase in the amount of bridge material transfer, the contact surfaces can be excessively deformed and arcs may be produced, resulting in excessive growth of well-known tarnish films and in imperfect contact or other malfunctioning.

Since bridge material transfer in working electric contacts invariably shorten the life of contacts, the present inventor, in an effort to suppress bridge material transfers as much as possible, has engaged in research for several years using experimental models of various multilayer electrical contacts, notably those employed in reed switches which were operated in test circuits employing arc quenching means before being made aware of the fact that the selection of a combination of two particular metals for plating on the base metal or alloy of the contact will provide a promising and reliable solution, provided that the two metals meet the following conditions:

(1) One of the metals employed should have a relatively small heat conductivity and a relatively high melting point as compared with other metal employed in the multi-layer structure.

(2) The two metals should not be diffused into one another in the fabrication of the multi-layer contacts, nor should they be diffused in the operating condition.

Whereas rhenium-gold (Re-Au) combination has been proven to be effective for suppressing the possibility of growth of buildup in electrical contacts, as will be made clear from a consideration of the life test result shown in FIG. 2, the present inventor is convinced that other combinations of metals will prove to be equally effective in that the previously mentioned conditions can be met.

The metals which are believed to yield the desired results are classified in a first group consisting of rhenium and tungsten and a second group consisting of gold, silver and copper. Selecting one metal from each of the groups in the formation of the multilayer contact, therefore, appears to provide the desired objective and results.

Accordingly, a principal object of the invention is to provide multilayer electric contacts, notably of those types employed in reed switches, in which possibility of occurrence of bridge material transfer is reduced to a minimum, yielding structures characterized by improved current-carrying capacities, improved contact reliability at moderate load currents, and longer service life in line with their relatively compact design.

Another object of the present invention is to provide novel multilayer electric contacts, preferably of the reed switch type in which at least one of the contacts is provided with a plurality of alternating layers of metallic materials wherein the outermost layer is comprised of a metallic material having a relatively small heat conductivity and a relatively high melting point as compared with the next adjacent layer to provide for even Wearing contact-engaging surfaces, even after repeated contact closures.

These as well as other objects of the present invention will become apparent when reading the accompanying description and drawings in which:

FIG. 1 is a schematic diagram of a circuit employed for conducting life tests upon reed switches employing the multilayer contact structure of the present invention.

FIG. 2 is a graph of curves in which the number of operations are plotted against cumulative failure rate of a typical reed switch having a multilayer contact structure of the type characterized by the present invention, and of a conventional reed switch having a multilayer diffused nickel-gold contact structure wherein the tests were performed employing the test circuit of FIG. 1.

FIG. 3 shows one preferred embodiment of the electric contact structure of the present invention and FIG. 3a shows a detailed view of a portion of the contact structure of FIG. 3.

Metallographic studies were conducted to explore the physical mechanism underlying the favorable operating behavior of multilayer electric contacts designed in accordance with the principles of the present invention during which numerous observations were made. The mechanisms for suppressing the growth of buildup associated with the formation of pits with the improved multilayer contacts will now be outlined in connection with a preferred embodiment of the invention.

According to the preferred embodiment as shown in FIG. 3, the overlapped portions of two reeds l1 and 12 made of nickel-iron alloy (conventionally employed in reed switches of the type identified in the United States as G-29) are each plated with four successive metal coatings Ilia-13d and .l4a-l4d, respectively, wherein the coatings are comprised of two types of metals, namely, rhenium (Re) and gold (An), in such a manner that the outermost layers 13a and 14a and the third layers 13c and 1410 are rhenium and the second layer 13b-14b and fourth (or innermost) layer l3d-l4d are gold.

Since the overlapped portions of the reeds ll and 12 constitute a pair of similar contact structures using simi- J1me TAKESKI SASAMOTO 3,51,3@

MULTILAYER ELECTRI C CONTACTS Filed July 19, 1968 Q R mg Z 5 3 {3 2.7 N E Although this invention has been described with respect to its preferred embodiments, it should be understood that many variations and modifications will now be apparent to those skilled in the art:

The embodiments of the invention in which an exclusive privilege or property is claimed are defined as follows:

1. A pair of selectively engageable electrical contacts;

each of said contacts including a base comprised of a suitable metallic material; at least one of said contacts having at least three alternately different metal layers mounted upon said base wherein the outermost and innermost layers are each formed of a metallic material having a relatively small heat conductivity and a relatively high melting point as compared with the metallic material employed as the layer interspersed between the outermost and innermost layers;

the remaining one of said contacts having a layer deposited upon its base, said layer being formed of the same material as the outermost layer of said one of said contacts.

2. The electrical contact structure of claim 1 wherein said outermost layers are formed of a metal taken from the group consisting of rhenium and tungsten, and wherein the interspersed layer is formed of a metal taken from the group consisting of gold, silver and copper.

3. The contact structure of claim 1 wherein said layers are plated upon their associated bases.

4. The contact structure of claim 1 wherein said layers are welded upon their associated bases.

5. The contact structure of claim 1 wherein an additional layer of metallic material is positioned between the innermost layer and said base to facilitate bondage of the mulilayer structure to the metallic base.

6. A pair of selectively engageable electrical contacts each being comprised of a base member formed of a suitable metallic material;

at least three alternately diiferent metallic layers mounted upon the base member of each of said contacts wherein the outermost and innermost layers are formed of a first metallic material and wherein the intermediate layer is formed of a second metallic material;

said first metallic material having a relatively small heat conductivity and relatively high melting point as compared with the second metallic material.

7. The electrical contact structure of claim 6 wherein the first metallic material is a metal taken from the group consisting of rhenium and tungsten, and wherein the second metallic material is taken from the group consisting of gold, silver and copper.

References Cited UNITED STATES PATENTS 2,387,903 10/ 1945 Hensel. 3,078,562 2/ 1963 Gwyn. et al. 3,125,654 3/1964 Arnold. 3,214,558 10/1965 Huber. 3,249,728 5/1966 Sasamoto et al.

HERMAN O. JONES, Primary Examiner 

